demangle: Parse optional single template argument for built-in types

The optional single template argument for built-in types is not documented in LLVM's comments that list the grammar, but it is [intentionally implemented](llvm/llvm-project@4bc4d51).

PiperOrigin-RevId: 606843797
Change-Id: Ib5ef1983812e9ea68a39fe64b87b6ad6ce81b93c

Author: dinord

absl/debugging/internal/demangle.cc

@@ -1297,32 +1297,39 @@ static bool ParseCVQualifiers(State *state) {
 }
 
 // <builtin-type> ::= v, etc.  # single-character builtin types
-//                ::= u <source-name>
+//                ::= u <source-name> [I <type> E]
 //                ::= Dd, etc.  # two-character builtin types
 //
 // Not supported:
 //                ::= DF <number> _ # _FloatN (N bits)
 //
+// NOTE: [I <type> E] is a vendor extension (http://shortn/_FrINpH1XC5).
 static bool ParseBuiltinType(State *state) {
   ComplexityGuard guard(state);
   if (guard.IsTooComplex()) return false;
-  const AbbrevPair *p;
-  for (p = kBuiltinTypeList; p->abbrev != nullptr; ++p) {
+
+  for (const AbbrevPair *p = kBuiltinTypeList; p->abbrev != nullptr; ++p) {
     // Guaranteed only 1- or 2-character strings in kBuiltinTypeList.
     if (p->abbrev[1] == '\0') {
       if (ParseOneCharToken(state, p->abbrev[0])) {
         MaybeAppend(state, p->real_name);
-        return true;
+        return true;  // ::= v, etc.  # single-character builtin types
       }
     } else if (p->abbrev[2] == '\0' && ParseTwoCharToken(state, p->abbrev)) {
       MaybeAppend(state, p->real_name);
-      return true;
+      return true;  // ::= Dd, etc.  # two-character builtin types
     }
   }
 
   ParseState copy = state->parse_state;
   if (ParseOneCharToken(state, 'u') && ParseSourceName(state)) {
-    return true;
+    copy = state->parse_state;
+    if (ParseOneCharToken(state, 'I') && ParseType(state) &&
+        ParseOneCharToken(state, 'E')) {
+      return true;  // ::= u <source-name> I <type> E
+    }
+    state->parse_state = copy;
+    return true;  // ::= u <source-name>
   }
   state->parse_state = copy;
   return false;

absl/debugging/internal/demangle_test.cc

@@ -150,6 +150,38 @@ TEST(Demangle, FunctionTemplateTemplateParamWithConstrainedArg) {
   EXPECT_STREQ(tmp, "foo<>()");
 }
 
+TEST(Demangle, NonTemplateBuiltinType) {
+  char tmp[100];
+
+  // void foo(__my_builtin_type t);
+  //
+  // foo({});
+  ASSERT_TRUE(Demangle("_Z3foou17__my_builtin_type", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "foo()");
+}
+
+TEST(Demangle, SingleArgTemplateBuiltinType) {
+  char tmp[100];
+
+  // template <typename T>
+  // __my_builtin_type<T> foo();
+  //
+  // foo<int>();
+  ASSERT_TRUE(Demangle("_Z3fooIiEu17__my_builtin_typeIT_Ev", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "foo<>()");
+}
+
+TEST(Demangle, FailsOnTwoArgTemplateBuiltinType) {
+  char tmp[100];
+
+  // template <typename T, typename U>
+  // __my_builtin_type<T, U> foo();
+  //
+  // foo<int, char>();
+  ASSERT_FALSE(
+      Demangle("_Z3fooIicEu17__my_builtin_typeIT_T0_Ev", tmp, sizeof(tmp)));
+}
+
 // Test corner cases of boundary conditions.
 TEST(Demangle, CornerCases) {
   char tmp[10];